The present invention relates generally to chlor-alkali electrolytic cells employing flowing mercury as the cathode. More specifically, the present invention relates to a protective shield that is placed adjacent the electrolyzer cover to prevent the retainers which fasten the cover to the cell from being propelled at high speed across the cell building area in the event that hydrogen gas buildup within the cell gas rises to an explosive state. The protective shield also prevents hot brine from being expelled from the electrolytic mercury cell container onto the work area where operators may be in the event of an explosion.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chlor-alkali cells. The chlor-alkali cells are of two general types, the deposited asbestos diaphragm-type of electrolytic cell or the flowing mercury cathode-type of electrolytic cell. In the mercury cells, the brine is electrolyzed between anodes of a particular composition and the flowing mercury cathode. Chlorine is liberated at the anodes and sodium amalgam is formed at the cathode. In a subsequent step, caustic soda and hydrogen are produced from this amalgam in a separate decomposer compartment. Because of this two-step operation, mercury cells are able to produce without further treatment an unusually pure grade of 50% caustic that has exceptionally low sodium chloride content. Frequently, because of this low salt content, the mercury cell caustic does not require further purification prior to industrial use. Thus, the caustic, as well as the amalgam and the chlorine, produced from mercury cells are still economically attractive products.
There have been numerous advances in the mercury cell technology, such as the computer adjustment of anodes during operation to optimize efficiency of cell operation. However, because of the nature of the operation of mercury cells, the percentage of hydrogen in the cell gas continues to be a troublesome controlling factor in mercury cell operation. A mixture containing more than 5% hydrogen in chlorine is explosive if exposed to sunlight or an electrical discharge. The concentration of hydrogen in chlorine is normally about 0.2 to 0.5 percent. The release of energy from an explosion caused by hydrogen buildup above the acceptable level, often as low as 1 to 3 percent by volume, can cause the cell or electrolyzer cover to warp or bend, shoot off the C-clamps that hold the cover in place, and cause hot brine to spill over the sides of the cell container onto the operators who are working in the area.
This buildup of hydrogen gas can be caused by a number of different conditions. Impure brine containing magnesium, iron and other polyvalent metals in suspended solids can cause the hydrogen overvoltage on the mercury surface to lower and produce excessive hydrogen discharge during the operation of the cell. A low brine concentration or an inadequate supply of brine can also lead to abnormally rapid formation of hydrogen in the mercury cell. The high sodium concentration in the amalgam, normally anywhere above 0.2 percent, can cause the percentage of hydrogen in the chlorine stream to increase. This is especially true if coupled with other unfavorable operating conditions. Mechanical difficulties in the operation of the cell, such as a stoppage of mercury circulation across the sloped bottom of a mercury cell, the breakage of an anode, or a short circuit to the mercury cathode, can also result in a rapid buildup of hydrogen gas. Should the bottom of the mercury cell or electrolyzer be badly out of level so that the flow of the liquid is disrupted, rapid hydrogen gas buildup will also occur. Similarly, excessive accumulation of impurities in the bottom of the cell will have the same effect. Lastly, the pH of the brine employed can affect hydrogen buildup. It is known that hydrogen formation is markedly accelerated when the pH drops below 1.2.
Other occasions when explosive potential can be realized occur when there is a shutdown of mercury cells. Whenever the current is interrupted, the sodium in the amalgam reacts with water in the electrolyzer cell to form hydrogen, even though no chlorine is produced. If one cell is stopped, it must be immediately vented or, if a plant-wide power failure occurs, emergency power must be provided to maintain suction in the chlorine gas conduits to remove hydrogen from the cell. The start-up of these cells can be particularly hazardous if air is not drawn through the cells prior to start-up after shutdown. Since most commercial mercury cells operate in approximately a 260,000 to 300,000+ ampere operating current range with approximately 4+ volt DC potential, any buildup of hydrogen gas from the aforementioned condition makes explosions a distinct and likely possibility.
These explosions can warp the electrolyzer covers or cell tops, even though they are normally formed from up to 1/2 inch thick metal. Additionally, C-clamps that hold the covers or tops in place are shot off by the explosive force and can be propelled considerable distances through the cell building. Explosions from localized pockets of hydrogen gas will also cause the hot brine in the electrolytic containers to spill over the sides of the cells, presenting definite safety hazards to operators in the immediate area.
The foregoing problems are solved in the design of the apparatus comprising the present invention.